1. Field of Invention
This invention relates to automatic control of heating, ventilating, and air conditioning (HVAC) systems and perimeter heating systems providing heat to a temperature regulated space having occupied and unoccupied periods, and more particularly to automatic control of HVAC and perimeter heating systems during unoccupied periods of the space.
2. Description of the Prior Art
As may be known, HVAC systems are used for regulating the temperature in one or more living spaces within a building enclosure. Similarly, as may be known, perimeter heating systems are used to provide supplemental heating to that provided by the HVAC system in those living spaces located along the perimeter of the building, which are subject to more severe ambient temperature changes due to the "cold wall effect" produced by outside temperature and wind conditions. The HVAC systems may be of a variety of well-known types, such as the multi-zone, dual duct, or terminal reheat type systems, each having an air handler source which includes individually controlled heating and cooling apparatus for conditioning mixed air presented to the air handler from a mixed air chamber. The mixed air chamber provides the mixed air as a combination of outside air and return conditioned air from the space, the ratio of each being determined by position modulation of mixed air dampers within the chamber. Each type of HVAC includes at least one system fan, having a high horse power rating as required for forcing a large volume of mixed air through the air handler and associated hot and cold decks into the space. Although the heating and cooling apparatus account for a major portion of the energy consumed by the HVAC during operation, the fans account for a substantial portion of the total HVAC energy consumption, and provide the highest electrical energy consumption device in the system.
The perimeter heating system includes a hot source for providing a hot fluid, such as hot water, to heaters installed in the perimeter living spaces. A pump provides circulation of the hot water through the spaces serviced by the system, back to the hot source. As with the HVAC fan, the perimeter heating system pump is a high horsepower device as required to provide a sufficient hot water flow rate through the system, and as such accounts for a substantial portion of the total energy consumption of the perimeter heating system.
In general, the living spaces serviced by the HVAC and perimeter heating systems have scheduled human occupancy periods, resulting in unoccupied periods extending over hours or days. During the unoccupied periods it is desirable to relax the space ambient temperature requirements to heating and cooling reference temperature values which are lower and higher, respectively, than those temperatures required for human comfort, in order to conserve energy. The space ambient temperature during the unoccupied periods must be high enough to protect against freezing of the space in the event of sudden drops in the outside ambient temperature, and low enough to protect against overheat due to either self-heating, or an increase in outside temperature. At present, a number of systems are known in the art for providing "set-back", or unoccupied, reference temperatures for controlling the HVAC operation during the unoccupied period of the space. These prior art systems provide the set-back references at a lower heating reference temperature and a higher cooling temperature than that provided during occupied periods, however, due to system limitations the set-back temperature references are selected at conservative values above and below the freezing point and overheat temperatures to ensure the protection of the space. As a result the temperature differential between the set-back heat and cool reference temperature does not differ substantially from the temperature differential between the corresponding occupied reference values, such that although energy conservation is realized, the fully realizeable energy savings is not achieved. The prior art systems make no provision for controlling the operation of the HVAC fan, i.e. turning on the fan only as required to provide air circulation through the air handler and living space when either the HVAC heating or cooling apparatus is on during the unoccupied period. The fan is generally left on throughout the unoccupied period in anticipation of required mechanical heating or cooling to the set-back reference temperatures, and is shut down only in the event of a complete system shutdown. Similarly, the prior art systems do not provide selective turn on of the perimeter heating system hot source or actuation of the pump, each remaining on or fully shut down, depending upon the anticipated outside ambient conditions during the unoccupied period.
As a consequence of the set-back temperature regulation of the space during the unoccupied periods, there is a requirement that the space ambient temperature be within the occupied temperature reference limits prior to human occupancy in the space. Generally, the prior art systems ensure that the ambient temperature is at the occupied reference limits by providing an extended start-up, or pre-heat period within the latter portion of the unoccupied period during which the HVAC is turned on, and controlled to the occupied temperature reference values to provide the required occupied ambient temperature conditions. As may be appreciated, the longer the start-up period, the shorter the total time interval for the set-back period, and consequently the less effective the energy conservation. A typical prior art system for controlling the unoccupied space temperature to conserve energy and to provide a desired occupied temperature at a predetermined occupied time, is disclosed in a patent to Davis et al, U.S. Pat. No. 3,979,059, wherein the space temperature is increased in a "pre-heat period" from a set-back temperature reference to an occupied temperature reference in dependence on the temperature conditions within the space and outside the building. The heating apparatus is controlled to the differential temperature between the occupied and unoccupied set point references and is modified in dependence on the outside air temperature. A provision is also made for an extended pre-heat period following a longer than usual off period, i.e. a weekend. Through the use of both inside and outside air temperatures the Davis system provides the required occupied ambient conditions prior to the end of the unoccupied period, however, the pre-heat required is shown to be a substantial portion of the unoccupied period. In the case of a normal unoccupied period, i.e. overnight during the week, the required pre-heat period is typically fifty percent of the total unoccupied period, which limits substantially the amount of energy saving provided by the set-back control procedure.
In addition, the prior art systems, including that of Davis, do not consider outside wind conditions in establishing the time duration of the pre-heat period. As may be known, the effect of wind on the ambient temperature conditions within the perimeter living spaces is substantial, the wind contributing to the cold wall effect, and the ambient temperature of the perimeter spaces being directly proportional to the average wind speed. As a result the interior living spaces may be provided with suitable ambient temperature conditions prior to occupancy, while the perimeter spaces may be at a significantly lower ambient temperature value.